Oxygen is essential on every dive, and it can harm you without warning. At the surface it is harmless. Under pressure it is toxic: to the brain at high partial pressures, and to the lungs over long exposures.
Divers control oxygen exposure with 2 measures. The first is a pressure limit, the highest partial pressure of oxygen you may breathe. The second is a pair of clocks that record how much exposure you have accumulated. Both affect the choice of decompression gas.
- Air
Partial pressure and depth
Air is 21 % oxygen at any depth. The partial pressure of that oxygen, its share of the total pressure you breathe, increases with depth. This is Dalton’s law: fraction multiplied by pressure.
At the surface, air gives about 0.21 bar of oxygen. At 10 m the pressure has doubled, so the partial pressure doubles too. Your body responds to the partial pressure, not to the percentage on the cylinder label.
The 1.4 bar working limit
Nitrox contains more oxygen than air, so it reaches a given ppO₂ at a shallower depth. Most divers keep the gas they swim and work on at or below 1.4 bar.
The depth at which a mix reaches that limit is its maximum operating depth (MOD). The engine marks it on each line. Richer mixes give longer no-stop times but have a shallower MOD.
The 1.6 bar decompression limit
During decompression you are at rest and breathing easily, so a higher limit is accepted: 1.6 bar, the highest value in the NOAA table. This limit is why EAN50 is used from 21 m and pure oxygen from 6 m.
Work, cold and a build-up of carbon dioxide all increase the risk of oxygen toxicity. For this reason the limit on the bottom is lower than the limit on a stop.
Types of oxygen toxicity
In 1878 Paul Bert showed that animals breathing oxygen under pressure had convulsions. About 20 years later Lorrain Smith found that long exposures at lower pressures caused inflammation of the lungs. The 2 effects are still named after them. Divers track them separately because each depends on pressure and time in a different way.
- Minutes allowed (CNS)
The CNS clock
Central nervous system (CNS) toxicity is the more dangerous of the two. It can cause a convulsion with little or no warning. Underwater, a convulsion can lead to loss of the mouthpiece and drowning.
The US National Oceanic and Atmospheric Administration (NOAA) publishes how long a diver may breathe each ppO₂ in a single exposure. The CNS clock is the percentage of that limit used so far. For example, 75 minutes at 1.4 bar, against a 150-minute limit, is 50 %. At 1.6 bar the limit is 45 minutes.
The pulmonary clock: OTU
Lung damage develops slowly, from the dose accumulated over hours and days. It is measured in oxygen tolerance units (OTU). One OTU is 1 minute of pure oxygen at 1 bar. Exposure below 0.5 bar does not count.
As ppO₂ rises, the OTU rate increases much more slowly than the CNS clock. A short, deep dive on an oxygen-rich gas mainly adds CNS exposure. Several days of long nitrox dives mainly add OTU.
Both clocks on a planned dive
The figure shows both clocks through a dive to 40 m for 25 minutes on air, with EAN50 from 21 m. On the bottom, air at 40 m gives about 1 bar of oxygen, so both clocks rise slowly.
At the switch to EAN50 the ppO₂ rises to about 1.6 bar. Most of the oxygen dose on this dive is taken on the stops.
Deco gas and oxygen dose
The grey lines show the same dive with no deco gas. A higher oxygen fraction means a lower nitrogen fraction, so the tissues release nitrogen faster. Without EAN50 the stops take much longer.
Both clocks, however, finish lower. A rich deco gas shortens decompression and increases oxygen dose. Divers choose deco gases and switch depths to use that dose where it shortens decompression most.
Try it
Set a ppO₂ and a time. The engine returns both clocks for that single exposure, with marks at the values where DiveLogic’s planner warns. Compare 1.4 bar for 60 minutes with 1.6 bar for 60 minutes.
What to remember
- Oxygen toxicity depends on partial pressure: the oxygen fraction of the gas multiplied by the pressure at which you breathe it.
- Most divers keep working gases at or below 1.4 bar and use 1.6 bar only at rest on decompression.
- The CNS clock is the share of NOAA’s single-exposure limit used; it falls from 150 minutes at 1.4 bar to 45 at 1.6.
- OTU counts the slower lung dose and does not reset at the surface over a dive day.
- A rich deco gas shortens decompression but increases oxygen dose. Balancing the two is one of the main decisions in a technical dive plan.
Oxygen exposure in DiveLogic
DiveLogic’s CNS and OTU calculator uses the same engine call as the figure above. Every plan in the planner shows both clocks at each step and warns at 80 % CNS and 200 OTU. The gradient factors guide covers the decompression side.
Sources
- Bert P. (1878). La pression barométrique: recherches de physiologie expérimentale. Masson, Paris.
- Smith J. L. (1899). The pathological effects due to increase of oxygen tension in the air breathed. Journal of Physiology 24(1): 19-35.
- Donald K. W. (1947). Oxygen poisoning in man. British Medical Journal 1: 667-672 and 712-717.
- Clark J. M., Lambertsen C. J. (1971). Pulmonary oxygen toxicity: a review. Pharmacological Reviews 23(2): 37-133.
- NOAA (2001). NOAA Diving Manual: Diving for Science and Technology, 4th edition. Best Publishing, Flagstaff, AZ. (Oxygen exposure limits.)